(1) Field of the Invention
The present invention generally relates to a fuel tank pressure control apparatus, and more particularly to an apparatus for controlling an internal pressure of a fuel tank in an evaporated fuel purge system when an engine is operating and when the engine operation is stopped.
(2) Description of the Related Art
In an internal combustion engine of an automotive vehicle, an evaporated fuel purge system is provided. In the evaporated fuel purge system, fuel vapor evaporated in a fuel tank is supplied to a canister so that the fuel vapor is absorbed by an absorbent in the canister. The fuel vapor in the canister is fed into an intake passage of the engine through a purge passage when the engine is operating under prescribed operating conditions.
In the evaporated fuel purge system, if a large amount of fuel vapor is supplied to the canister via a vapor passage, or if the amount of the supplied fuel is greater than the permissible amount of fuel that the canister can store (or the canister capacity), the canister may overflow with the fuel supplied, and the excessive fuel may escape from an opening of the canister into the atmosphere.
In order to eliminate the above described problem, it is necessary to regulate the amount of fuel vapor evaporated in the fuel tank at a reasonable level. For this purpose, in a conventional evaporated fuel purge system, means for setting the internal pressure of the fuel tank to a relatively high pressure is provided.
When the engine has just stopped operating, the fuel in a sub-tank of the fuel tank is still at a relatively high temperature. The fuel in a main tank of the fuel tank is subjected to the heat of the fuel in the sub-tank and to the heat of an exhaust system surrounding the fuel tank. Thus, the fuel temperature is temporarily increased after the engine has stopped operating, and an increased amount of fuel vapor is produced in the fuel tank. Such fuel vapor is supplied from the fuel tank to the canister.
However, no fuel vapor from the canister is supplied to the intake passage of the engine after the engine has stopped operating. Accordingly, there is a problem in that the increased amount of fuel vapor produced in the fuel tank after the engine has stopped operating is supplied to the canister. Thus the canister may overflow.
Japanese Laid-Open Utility Model Publication No. 51-105906 discloses a pressure control valve arranged in a passage between a fuel tank and a canister. The control valve is closed when the engine stops operating, so as to increase the internal pressure of the fuel tank preventing a large amount of fuel vapor from being produced when the engine stops operating. The control valve has a bypass passage which is open to the fuel tank when the engine is operating, allowing the internal space of the fuel tank to communicate with the canister via the bypass passage.
However, in the pressure control valve disclosed in the above mentioned publication, the fuel tank is open to the canister via the bypass passage when the engine is operating. Thus, it is difficult to prevent the fuel in the fuel tank from spilling to the canister via the pressure control valve when the vehicle is turning around.
Also, in the pressure control valve disclosed in the above mentioned publication, a cut-off valve for preventing the fuel tank from being replenished with an excessive amount of fuel is provided. However, the cut-off valve is inoperative if the fuel supply is gradually increased, and it is difficult to prevent the fuel tank from being filled with excessive amount of fuel.